There are a wide variety of applications where process or transport tubing becomes fouled with deposits or scale. Water jets, generated by a rotating jetting tool and directed across the internal surface of the tubing or pipe, are commonly used for cleaning these deposits. Such rotating jetting tools can also be used to drill through soil and rock formations. The jet quality provided by the rotating jetting tool is important, especially in harder formations. Jet quality is affected by a number of factors, including standoff distance and upstream flow conditions. Orienting the discharge nozzles of the tool at a large angle relative to its axis of rotation reduces jet standoff distance and improves jetting performance. Uniform upstream flow channels improve jet quality by reducing turbulence intensity. Many designs for rotating jetting tools incorporate relatively small fluid passages, which reduce the pressure and power available for jetting. Other systems require that the operating fluid used be filtered to a high degree, which adds significant expense and complexity. It would be desirable to provide a rotary jetting tool with relatively large flow passages, which does not require the use of an extensively filtered operating fluid.
Rotating jetting tools may use an external motor to provide rotation, or the rotor can be self-rotating. A self-rotating system greatly simplifies the tool operation. In a typical self-rotating system, the jets of liquid are discharged with a tangential component of motion, which provides the torque necessary to turn the rotor. Most self-rotating systems use a sliding seal and support bearing to enable the rotation of the working head. The drawback to this configuration is that the torque produced by the working jets must be sufficient to overcome the static bearing and seal friction. The dynamic friction of bearings and seals is typically lower than the static friction, so once the rotor has started to turn, it can spin at excessive speeds, which can cause overheating or bearing failure. It would be desirable to provide a rotary jetting tool that is configured to prevent such excessive rotation.
Most self-rotating jetting systems also incorporate a thrust bearing to counteract the internal pressure of the fluid against the nozzle. These bearings are subject to high loads and can fail when the rotor's rotational speed is excessive. The thrust load can be eliminated with a balanced or floating rotor design, wherein the shaft is supported by opposed radial clearance seals. If the shaft diameter is the same on both ends of the rotor, there is no thrust due to the internal pressure of the fluid. The clearance seals also act as hydrodynamic journal bearings, which rely upon a thin film of fluid that supports the rotating shaft using hydrodynamic forces. While journal bearings cannot support high thrust or radial loads, they are effective at high velocity—where the hydrodynamic support is greatest.
This approach has been used by Schmidt (as disclosed in U.S. Pat. No. 4,440,242) and Ellis (as disclosed in U.S. Pat. No. 5,685,487) to achieve a self-rotating jet. In the Ellis design, the working fluid is introduced from the tangential surface of the rotor shaft to the center of the rotor by crossing ports. One drawback to this configuration is that the fluid settling chamber is small compared with the sealing diameter of the rotor. In the Schmidt patent, the jet rotor extends well beyond the thrust-balanced section and can be relatively large.
The greatest drawback to the use of radial clearance seals is that clearance seals are prone to jamming with debris, especially when the operating pressure is applied slowly. Sealing, for this approach, is accomplished by maintaining a small clearance, or gap, between the inner and outer elements of the rotor, and leaving a small leakage path for the fluid. Particles approximately the same size or larger than the gap can easily get jammed in the gap and can build up during periods when fluid pressure is low and the rotor is not spinning. When the fluid pressure is increased, such particles are jammed even tighter into the gap and will then prevent the rotor from spinning freely. To avoid this problem, the working fluid must be filtered to remove all particles that might obstruct the smallest gap in the rotor head. Because the gaps must be small to prevent excessive fluid leakage, the fluid must again be filtered to a high degree. In many applications, a relatively large volume of working fluid is required, and filtering the fluid becomes impractical. It is also desirable to be able to pump abrasives or other particles through a jet rotor to enhance the jetting process.
Mechanical face seals overcome the problem of debris jamming the sealing gap. The nominal gap between the sealing surfaces is zero, and leakage is zero when the rotor is not rotating. If fluid is not flowing through the gap, debris cannot be carried into it. Secondly, the sealing gap is not rigidly fixed, as in a radial clearance seal. One element of a mechanical face seal is spring loaded and pressure activated with a secondary seal. If, for some reason, a particle were conveyed into the gap between the sealing faces, the sealing faces can spread, enabling the particle to pass through. Thus, particles are unlikely to become stuck in the sealing gap, and if they do, such particles can escape from the gap as a result of this self-clearing action.
The use of pressure-balanced mechanical face seals for fluid pumping applications is well known in the art. The most common application of mechanical face seals is to provide a fluid seal around a rotating shaft where the shaft penetrates a pressurized vessel so that the fluid is retained in the vessel and does not leak out of the vessel around the shaft. In most cases, such as in single-stage centrifugal pumps, the end of the shaft is exposed to an elevated pressure. This pressure, multiplied by the effective sealing area, produces an end load on the shaft to which a thrust bearing must react. In most pump applications, external support bearings can be provided to withstand the thrust. A mechanical face seal includes a rotating seal ring with a face that slides on a static seal ring. The rotating seal ring is keyed to rotate with the shaft, and is provided with a static seal element that can slide along the shaft. Pressure forces on the rotating element force it axially into contact with a static seal element that is attached to the pressurized vessel. As long as the contact force is greater than the pressure within the pressurized vessel, the seal is effective. The contact force between mechanical sealing faces is determined by the balance ratio of the seal. The balance ratio represents the ratio between the sealed area and the area on which the average pressure between the seal faces acts. This ratio can be adjusted by controlling the seal ring contact area and diameter of the static seal between the rotating seal ring and the shaft. Since the average pressure between the seal faces is normally about one-half the sealed pressure, the seal head will be in equilibrium for a balance ratio of 0.5. It is common practice to choose a balance ratio from 0.65 to 0.75 for contacting face seals. High pressure results in high contact forces between the seal faces, which can lead to premature failure and a high starting torque.
Conventional mechanical face seals have not been used in high-pressure rotating jetting tools for a variety of reasons. The high operating pressure imposes a high shaft end load, which is the product of the operating pressure and the area of the rotating shaft that is sealed. In a conventional design, the shaft load is supported by separate thrust bearings, and the pressure is sealed with a mechanical face seal. The need for separate thrust bearings complicates the tool design and increases the length of the jetting tool. Secondly, the high-operating pressure imposes high contact loads on the seal faces, which results in a high starting torque. The most convenient mechanism for imparting a rotational force to a rotating jetting tool is to use the reaction torque generated by offset jets. This torque is relatively small and is generally insufficient to overcome the friction torque of a conventional mechanical face seal. Finally, it may be desirable to operate rotating jetting tools at relatively high rotational speeds, resulting in a high pressure-velocity (PV) load on any conventional mechanical face seal included within the rotating jetting tool. The PV relationship is defined as the product of contact stress and sliding velocity. High PV values cause premature wear and failure of mechanical face seals. For rotors used in rotating jetting systems for drilling and servicing oil and gas wells and production equipment, an external thrust bearing is impractical, and the thrust loads must be much lower than those induced by the working pressure multiplied by the effective seal area. It would thus be desirable to provide a rotor designed for use in rotating jetting systems for the oil and gas industry that provides the benefits of mechanical face seals, but without the disadvantages of mechanical face seals that were discussed above.